Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including volatile and non-volatile memory. Volatile memory can require power to maintain its information and includes random-access memory (RAM), dynamic random access memory (DRAM), and synchronous dynamic random access memory (SDRAM), among others. Non-volatile memory can provide persistent information by retaining stored information when not powered and can include NAND flash memory, NOR flash memory, read only memory (ROM), Electrically Erasable Programmable ROM (EEPROM), Erasable Programmable ROM (EPROM), phase change random access memory (PCRAM), resistive random access memory (RRAM), and magnetic random access memory (MRAM), such as spin torque transfer random access memory (STT RAM), among others.
Memory devices can be combined together to form a solid state drive (SSD). A solid state drive can include non-volatile memory, e.g., NAND flash memory and NOR flash memory, and/or can include volatile memory, e.g., DRAM and SRAM, among various other types of non-volatile and volatile memory. Flash memory devices, including floating gate flash devices and charge trap flash (CTF) devices using semiconductor-oxide-nitride-oxide-semiconductor and metal-oxide-nitride-oxide-semiconductor capacitor structures that store information in charge traps in the nitride layer, may be utilized as non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption.
Memory cells in an array architecture can be programmed to a desired state. That is, electric charge can be placed on or removed from the floating gate of a memory cell to put the cell into a number of stored states. For example, a single level cell (SLC) can represent two data states as represented by the binary units 1 or 0. Flash memory cells can also store more than two data states, e.g., 1111, 0111, 0011, 1011, 1001, 0001, 0101, 1101, 1100, 0100, 0000, 1000, 1010, 0010, 0110, and 1110. Such cells may be referred to as multi state memory cells, multiunit cells, or multilevel cells (MLCs). MLCs can allow the manufacture of higher density memories without increasing the number of memory cells since each cell can represent more than one digit, e.g., more than one bit.
MLCs can have more than one programmed state, e.g., a cell capable of representing two digits can have four programmed states, a cell capable of representing three digits can have eight program states, and a cell capable of representing four digits can have sixteen programmed states. Thus, MLCs can have 2B programmed states, where B is equal to the number of binary digits that can be stored in the cell. Conventionally, the programmed states directly represent data states. For example, a cell that can store two binary digits can be programmed to one of four programmed states directly corresponding to one of four data states, such as 11, 01, 10, and 00.